For example, a pressure sensor that is widely used is comprised of a diaphragm formed over a portion of a semiconductor chip and a piezoresistive element arranged in a bridge shape on the diaphragm, and pressure on a medium to be measured is sensed by converting the displacement of the diaphragm, corresponding to the pressure on the medium to be measured, into a change in resistance of the piezoresistive element (see Japanese Unexamined Patent Application Publication No. 2002-277337 (page 2, FIG. 2) (“JP '337”), for example).
In a sensor chip of a pressure sensor having a semiconductor piezo element such as cited in JP '337, and particular for a sensor chip in which a resistive elements configure a bridge circuit, aluminum or other type of metal wiring is disposed on the structure thereof, and the number of contacts between the resistance and the wiring is twice the number of resistors. However, in the case such as where heat is applied to a pressure sensor having this type of structure, (thermal stress) is due to the different coefficients of linear expansion among the aluminum wiring, insulating film and semiconductor resistances is generated in each of these constitutional elements, causing the zero point of the sensor output to shift.
On the other hand, on the (110) face of a 300 μm-thick silicon substrate of the pressure sensor cited in Japanese Unexamined Patent Application Publication No. H9-126922 (hereafter referred to as “JP '922”) a piezo element is formed by the thermal dispersion of boron, and electrode wiring is formed from diffused resistance in a manner similar to that by which the piezoresistive element is formed. Then, an aluminum layer is formed on the (110) face of the silicon substrate by means of an electron beam evaporation method, and this aluminum layer is patterned into a predetermined shape by means of a photolithographic process to form an electrode pad. Moreover, anisotropic etching with potassium hydroxide is performed on the rear surface side of the (110) face of the silicon substrate to form a diaphragm.
More specifically, in the pressure sensor cited in JP '922, as is evident from the drawings in that document, the (110) face of the silicon monocrystalline substrate is used to form the diaphragm and diffused resistance, and the formative direction of the piezoresistive element is aligned in approximately the same direction as the <111> orientation on the (110) face of the silicon substrate. Moreover, the wiring made from diffusion resistance has a configuration that faces only the <100> orientation and the <110> orientation on the (110) face of the silicon substrate. Having such a configuration, the wiring is provided with pressure sensitivity in an attempt to solve only the challenge of how to improve sensitivity, even if slightly. Thus, the pressure sensor cited in JP '922 has high pressure sensitivity, but does not have a configuration for solving the problem of achieving insusceptibility to micro-stress from the wiring and other stresses on the sensor.
Additionally, in the pressure sensor cited in JP '922, the semiconductor resistance is not protected by an insulating film, and therefore environmental durability is extremely poor, and high precision is not achieved due the large affect of noise components and drift in the sensor output.
An object of the present invention is to provide a pressure sensor having excellent environmental durability, high pressure sensitively, and being unaffected by micro-stress from the wiring and the like on the sensor.